Did you create an access rule on ISA to allow RPC dialogs between subnets and if yes what are the settings of this rule ?
Did you take a look at the ISA log to see if some traffic is blocked ?

Any network traffic that has to pass through an ISA server must be allowed by an access rule, so if you did not create any rule for that then RDP failure is absolutly normal.

The easiest way to check is failure comes from ISA blocking something is to create a new access rule on ISA that allow "all outgoing protocol" coming from address 172.30.1.233 going to address 172.30.2.56 and for "all users". The rule can be created fastly.
If this rule permits dialog between your machines then you can fine tune it to allow only necessary protocols (in your case RPC protocol should be enough) between desired IP ranges.

In all versions of ISA Server and the current version of FTMG, the default https protocol uses TCP port 443 and 563 only. This cannot be changed within the ISA or FTMG GUI and must be completed from a Windows cmd prompt on the ISA Server itself.
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